The invention relates to a decanter centrifuge comprising a rotatably journalled drum having at one end outlet openings for the separated liquid phase, and a conveyor rotatably journalled in the drum with a conveyor body containing an inlet in the form of a cavity for the feed to be separated, said inlet being radially defined by a wall coaxial with the conveyor body and provided with inlet apertures located between two adjacent flights of the conveyor and connecting the inlet with the space between the conveyor body and the internal side of the drum, the inlet being axially defined by a rotationally symmetrical first end wall and a second end wall located opposite the first end wall, said second end wall having a central projection extending towards the first end wall and containing an axial bore for an inlet pipe for the feed, said inlet pipe being coaxial with the drum, and having a mouth facing the inlet and being located in the plane perpendicular to the axis of the drum.
In decanter centrifuges of this type, it is a problem that the feed during acceleration to the angular velocity of the conveyor body receives twice as much energy as necessary for the liquid to form a liquid layer along the internal side of the inlet. The excess energy results in undesirable turbulent flows in the liquid extending from the inlet into the space between the conveyor body and the internal side of the drum where the energy is finally converted to heat.
The circumstance that excess energy is supplied to the liquid will be recognized by studying a unit volume of liquid present at the internal liquid surface of the inlet. This volume will have a kinetic energy given by EQU 1/2.rho..omega..sup.2 r.sup.2,
wherein .omega. is the angular velocity of the conveyor body, and r is radius to the overflow edge. The angular momentum L of the liquid volume about the axis of rotation is EQU .rho..omega.r.sup.2.
This angular momentum results from the influence of the inlet which rotates with the angular velocity .omega.. The energy supplied from the motor propelling the decanter centrifuge is thus EQU L.omega.=.rho..omega..sup.2 r.sup.2.
It will be seen that this energy is twice as large as the above stated energy that was necessary to keep the liquid volume in the free surface.
This excess energy cannot be deposited in the liquid or dissipated without giving rise to interfering liquid flows in the comparatively thin liquid layer on the internal side of the inlet, thereby decreasing the efficiency of the separation process.
U.S. Pat. No. 3,428,246 describes a decanter centrifuge of the above type where accumulation of solids in the inlet and resulting erosion of the inlet pipe is avoided by means of radial ribs on the first end wall in the peripheral area at the inlet openings, a second end wall shaped as an inclined baffleplate, a deflecter assembly on the inlet pipe and the projection on the second end wall, and outlet openings for the separated liquid located at a radius which is greater than the radius to the inlet openings.
EP patent application No. 0.177.838 describes a decanter centrifuge in which a flocculant is added to the feed in the area between the first end wall and the outlet openings. The flocculant is supplied under pressure through a nozzle and the feed flow is partly penetrated by the flocculant. The feed flow shown in the drawing, indicates that the outlet openings for the separated liquid is located radially further cut than the inlet openings in the inlet.
In a centrifuge described in FR patent No. 2.057.600 the outlet openings for the liquid phase are located radially inwards of the inlet openings, such that the liquid phase partially fills the inlet. The purpose is to effect separation of the solids within the inlet. In this centrifuge the second end wall is reduced to a set of spokes carrying one end of the tubular conveyor body, in order to permit the liquid phase to escape from the inlet directly to the outlet openings.